The ability to machine structures of ever-decreasing dimensions is governed by many factors. One such factor is the material's atomic properties with regard to its reduced dimensions. For example, working at the nanometer scale, devices derive their properties from the wave nature of electrons, which must be taken into account when machining. An atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscope commonly used to investigate nanostructures. It has a demonstrated resolution of fractions of a nanometer, which is more than 1000 times better than the optical diffraction limit. Information is gathered by “feeling” the surface's atomic forces with a mechanical probe. Piezoelectric elements facilitate tiny, but extremely accurate and precise movements by electronic or computer control.
The ability to fabricate nanostructures on substrates and repair photomasks at a nanometer scale is particularly desirous and has been facilitated by the advent of the AFM. A photomask is an opaque plate with holes or transparencies that allow electronic radiation energy, usually light, to pass through in a defined pattern. They are commonly used in photolithography, which is a process used in micro fabrication of microprocessors to selectively remove parts of a thin film or the bulk of a substrate. It uses the electronic radiation energy to transfer a geometric pattern from a photomask to an electromagnetic radiation sensitive chemical photo resist on the substrate. A series of chemical treatments then engraves the exposure pattern into the material underneath the photo resist. In a complex integrated circuit, for example, a Complementary Metal-Oxide-Semiconductor (CMOS) wafer will go through the photolithographic cycle up to 100 times and involve up to 100 photomasks (one for each layer).
Moore's law describes a long-term trend in the history of computing hardware. Since the invention of the integrated circuit in 1958, the number of transistors that can be placed inexpensively on an integrated circuit has increased exponentially, doubling approximately every two years The doubling was achieved mostly through the use of enhanced photolithography techniques employing photomasks. Over the past five decades the wavelength of the light source has been reduced to permit smaller feature size with photolithography, but the photomask complexity has also increased. As a result, photomask designers need ways to ensure repeatable and faithful reproduction of a photomask's pattern onto the substrate. Therefore, the most critical issue for the production of photomasks is controlling and eliminating pattern defects in the photomasks.
Integrated circuit designers are using methods called Reticle-enhancement techniques (RETs) to improve reproduction reliability. These techniques have been used with various exposure approaches, such as double-patterning and extreme-ultraviolet (EUV) technologies. One RET is optical proximity correction (OPC), in which subresolution changes to the shape of a feature greatly improve its printability. Smaller, more subtle, and increasingly unavoidable defects in the photomask's features can render expensive photomasks, or even an entire mask set, worthless.
The types of defects on the photomask that may need removal include trimming unwanted carbon patches, removing growth particles and correcting irregularly-shaped quartz bump defects. Currently, there are two options for photomask repair, Focused Ion Beam (FIB) or laser. While each technique has its advantages and unique capabilities, each has its particular limitations. Photomask repair technology has lagged well behind the capability requirements listed in the International Technology Roadmap for Semiconductors (ITRS). The ITRS is a set of documents produced by a group of semiconductor industry experts. These experts are representative of the sponsoring organizations which include the Semiconductor Industry Associations of the US, Europe, Japan, Korea and Taiwan.
Additionally, the need for sub-wavelength resolution has driven the implementation of phase-shifting photomasks for hyper-critical layer processing. The increased complexity of this layering technique has, in turn, dramatically increased photomask costs and cycle time. Advanced alternating phase shift photomasks may cost in excess of $10,000 per layer and take five or six times as long to produce as a standard photomask.
The production of even a single layer photomask for today's multicore microprocessors is a significantly difficult operation, and the results are not always optimal. Additionally, the time to produce, and quality check, a single layer photomask is long. If a layer of a photomask has to be “reshot”, the time and cost both go up exponentially. The machines that produce the photomask are expensive, so a fabricator usually schedules their machine for continuous fabrication, i.e., many photomask jobs, in order to recover their costs. If a rewrite of a photomask must be done, it will have adverse effects on the production schedule of the fabrication plant, which may mean missed deadlines and lost contracts. Therefore, it is extremely desirable to be able to repair any existing defects on the photomask after production.
Repair of photomask defects is quickly becoming a necessity. This means that the user of the photomask has neither the time nor the inclination to create a new photomask to replace a defective one. Even a miniscule defect on a photomask will render a microprocessor, created from the defective photomask, inoperable.
Material-subtractive repair technology nanomachining employs an application of atomic force microscopy (AFM). Nanomachining removes mask material, such as opaque defects, with no chemical residuals with unsurpassed depth control. Past technical challenges included poor repair-sidewall angles and poor shape definition in extremely small, high-aspect-ratio patterns. FIG. 1 illustrates the stress placed upon nanomachining tip 14 when removing a defect 11 within a photomask 10. The stress point 15 clearly illustrates the deformation of the nanomachining tip 14, which should be pyramid-shaped. This shape is usually chosen because of its three sided cross sectional properties.
It is therefore desirable to have a method for fabricating precise high aspect ratio nanometer structures, for example, to repair and rejuvenate photomasks used in photolithography, using nanomachining and atomic force microscopy that ensures less nanomachining tip deflection, provides unsurpassed depth control and provides better sidewall shaping. The present invention satisfies that need, as well as others, and overcomes limitations in conventional fabrication methods.